\documentclass[a4paper, 12pt]{article}

\begin{document}

If $f(x) = 3x + 7$ and $g(x) = x + 4$ then
\begin{equation}
f(x) + g(x) = 4x + 11
\end{equation}
and
\begin{equation}
f(x)g(x) = 3x^2 + 19x + 28.
\end{equation}

\end{document}
